1. Field of the Invention
The present invention relates to a radio equipment of portable type, particularly to a radio equipment of portable type made in one unit with an antenna and capable of reducing the influence of the things around the antenna upon the characteristics of the antenna.
2. Description of the Related Art
Recently, a radio equipment or terminal of portable type has become thinner and more compact. A radio equipment of portable type should be at present very easy to carry with. Therefore, thin and compactness of the equipment is very important features.
On the other hand, there is a problem that a human body affects an antenna mounted on radio equipment. A human body absorbs and scatters a radio wave of the frequency used in the radio equipment of portable type. Further, a human body fluctuates the antenna impedance used nearby. Generally, in respect of a high frequency band, a human body acts as dielectric media with a permittivity of 50 or higher. Consequently, a human body deteriorates the radiation characteristics of an antenna.
In addition, as the radio equipment of portable type becomes thin and compact, an antenna is positioned closer to the ear of the equipment user. This closer distance to the ear makes deterioration of the antenna characteristics more serious.
The inventors of the present invention measured the influence of a human body on the deterioration of the antenna characteristics, and obtained the results as shown in FIG. 1. The experiment used a radio equipment of portable type of 2 GHz and measured the influence of a human body on the antenna. FIG. 1 shows the deterioration of the antenna gain caused by the impedance fluctuation while the radio equipment is being operated, taking the equipment thickness as a parameter. As seen from FIG. 1, when the equipment thickness becomes thinner than 20 mm, the possibility of contacting the ear to the antenna increases, and when the antenna comes in contact with the ear, the antenna gain is largely deteriorated.
As mentioned above, the cause of this deterioration is the fluctuation in the antenna impedance due to the influence of a human body. A-human body has a very high dielectric constant. Thus, when the antenna is approached to a human body, the electrical length of the antenna is regarded as to be equivalently long. In this case, the antenna resonance frequency deviates from a desired value, and the antenna impedance changes accordingly.
Therefore, even if the antenna impedance is adjusted to the optimum level in the radio equipment of portable type itself, the antenna impedance will shift from the optimum value when it is brought close to a human body.
Further, when the antenna impedance is deteriorated, the transmission power of the radio equipment of portable type may be lowered. This problem will be explained hereinafter.
To radiate a radio wave from an antenna, the antenna must be powered first of all. The optimum condition to power the antenna is that the impedance of a feed line is the same value as the impedance of that antenna.
When the antenna impedance changes from the optimum value, the power supplied through the feed line is reflected by the input end of the antenna and returned to a transmitting amplifier. Therefore, the power outputted from the amplifier to the antenna is reduced.
Moreover the reflected power deteriorates the amplifier performance, such as gain and operation efficiency, and causes deterioration of the transmission performance of the radio equipment itself.
As a means of solving this problem, a method of examining the reflection coefficient between the amplifier and antenna has been proposed. This proposal includes the power amplifier disclosed by Jpn. Pat. Appln. KOKAI Publication No. 10-341117. This conventional power amplifier detects the current consumption in the power amplifier and the power reflected by an antenna, and supplies these detected signals to the control circuit. This control circuit sends a control signal to a variable phase snifter based on the both detected signals, and corrects phase shifts occurred between the power amplifier and antenna.
Since this prior art power amplifier corrects only phase shifts, the evaluation quantity changing range is narrow and the phase shifts can be corrected by measuring the evaluation quantities. However, if a shift has two vectors, that is, the phase and amplitude directions of a reflection coefficient, as in the antenna of a cellular phone, the correction becomes difficult by only the above two evaluation quantities. Moreover, the antenna impedance must be checked even while communication is not made. Otherwise, communication has to be started in bad conditions, causing a communication failure. In the radio equipment of portable type, it is resulted from the high probability of changing the surrounding conditions upon start of communication. For example, when a call comes in standby mode, a mobile terminal is taken out from a pocket, held by hand, set to conversation mode, and attached to a head. The conditions around the antenna are changed three times in a very short time.
The above-mentioned conventional method can check the impedance only while communication is being made in some types of the system. In the CDMA system, for example, only an incoming call can be accepted in standby mode, and the above-mentioned operation is impossible. Sending a radio wave unnecessarily except when making communication causes troubles in other systems, and is controlled by the Radio Law.
As explained above, there is a problem in the conventional radio equipment of portable type. Antenna characteristics are deteriorated by bad matching between a transmitter and an antenna, caused by the surrounding conditions, particularly coming close to a human body.